// #{$i} 把变量作为字符串。 $i 直接作为数值
@for $i from 1 through 4 {
    .col-#{$i} { width: 100/4 * $i + %;}
}

@each $usr in bob, john, bill, mike {
    .#{$usr}-avatar {
        background-image: url('/img/#{$usr}.png');
     }
}

$ppl: ( usr1:bob, usr2:john, usr3:bill, usr4:mike );

@each $key, $usr in $ppl  {
    .#{$usr}-avatar {
        background-image: url('/img/#{$usr}.png');
    }
}

$x:1;

@while $x < 13 {
    .col-#{$x} { width: 100/12 * $x;}
    $x: $x + 1;
};